Polypeptide factors which stimulate or inhibit growth have been identified in normal and neoplastic tissues and cells. A tumor inhibitory factor (TIF) has been identified from a human colon carcinoma cell line and from Novikoff rat ascites fluid. The TIF shows specificity in that it inhibits the growth of the more differentiated human cell lines tested but not the poorly differentiated cell lines. The goal of this proposal is to characterize TIF and examine its role in the control of cell growth. This will involve purification and characterization of TIF. The interaction of TIF with responsive and non-responsive cells will be determined and the TIF receptor in responsive cells identified by gel electrophoresis after crosslinking labeled TIF to the receptor in membrane preparations. The biological effects of TIF on responsive colon carcinoma cell lines will be determined. TIF could inhibit cell growth by acting as a cytotoxic or cytostatic agent or it could act as a differentiation agent. Experiments measuring mitosis, biosynthesis of macromolecules, and the ability of growth factors to induce mitogenesis of transformed cells will be used to distinguish between these alternatives. The reversibility of TIF action, the effects of dose and time of exposure to TIF, and interactions between TIF and transforming growth factors will be examined. The level of TIF production by responsive and non-responsive cells will be determined. This proposal will provide initial characterization of the molecular properties and the biological effects of a chemically defined inhibitory factor free of growth stimulatory activity. This will result in information concerning the mechanism of growth control in normal and transformed cells.